This article describes features and functioning of left-ventricular assist devices (LVAD). LVADs are the fastest-growing category of mechanical heart devices and are the only category of mechanical heart-assist devices to be approved in the United States, Canada, and Europe as a commercially available treatment. The current generation of LVADs and implantable artificial hearts represent a major technological step. However, by all accounts, these devices have a long way to go before they are as mechanically sound and user-friendly as they need to be to be widely accepted as a destination therapy for more than just the sickest of the sick. Thoratec Corp., Pleasanton, CA, has received Food and Drug Administration approval of its HeartMate SnapVE LVAD and an updated model, the HeartMateXVE, for use as a long-term, permanent treatment for end-stage congestive heart failure patients who don't qualify for heart transplantation and have a life expectancy of less than two years. These devices, the best known of which is the Abiocor, from AbioMed of Danvers, Massachusetts, are still considered experimental.
at its most basic) the heart is simply a pump. But, for nearly 4,000 Americans per year, that pump has broken down beyond repair. For those people, the best solution is a heart transplant. According to the American Heart Association, only about 2,000 of those on the Organ Procurement and Transplantation Network waiting list will get the new heart they're waiting for in any given year. For the other 2,000 people, however, there's hope on the horizon-in the form of new artificial heart devices and heart assist devices currently under development.
Recent developments in artificial heart technology have been progressing on two fronts: left-ventricular assist devices, which supplement a diseased heart; and fully implantable replacement hearts.
The assist devices, known as LVADs, are further along in the development process. Thoratec Corp. of Pleasanton, Calif, has received FDA approval of its HeartMate SnapVE LVAD and an updated model, the HeartMateXVE, for use as a long-term, permanent treatment for endstage congestive heart failure patients who don't qualify for heart transplantation and have a life expectancy of less than two years. World Heart Corp. of Ottawa, Ontario, has had its left ventricular assist system, the Novacor LVAS, approved for use as a bridge for transplantation. The company is currently seeking approval for use of the device as a destination therapy.
Fully implantable artificial hearts, which some consider the holy grail of mechanical heart assist technology, are still a long way from commercial approval. These devices, the best known of which is the Abiocor, from AbioMed of Danvers, Mass., are still considered experimental. As such, they are currently being offered only to the sickest of patients-those who have been determined to be ineligible for a heart transplant and who are not expected to survive longer than 30 days without treatment.
Left ventricular assist devices are the fastest-growing category of mechanical heart devices, and are the only category of mechanical heart assist devices to be approved in the United States, Canada, and Europe as a commercially available treatment.
An LVAD doesn't replace a diseased heart. Rather, it is implanted alongside the native heart and is designed to assist the pumping function of the heart's left ventricle. The left ventricle, which performs about 80 percent of the heart's work, supplies oxygenated blood to the entire body.
The Thoratec HeartMate Snap-YE LVAD and XVE and World Heart Novacor LVAS differ in some particulars, but operate on the same principles. All are electrically driven pumps that are implanted between muscle layers in the abdomen and attached parallel to the cardiovascular system. This leaves the diseased heart's connections intact, while providing the pumping power that the heart lacks. Blood is channeled into the assist device by way of an inflow conduit that is attached to the apex of the heart's left ventricle.
Once blood from the natural heart empties into the pump, an external control system triggers pumping. A pusher plate, forcing a flexible polyurethane diaphragm upward, pressurizes the blood chamber. This motion propels the blood through an outflow conduit and graft attached to the aorta, the main artery supplying the body with oxygen-rich blood. Valves located on either side of the device's pumping chamber keep blood flowing in only one direction.
Both the HeartMate devices and the Novacor LVAS are pulsatile devices that "beat" in conjunction with the patient's natural heart. Both devices are powered by rechargeable external battery packs that are worn in either shoulder or waist holsters. Both systems are monitored by' an external electronic controller. The controller and battery packs are connected to the implanted pump by a percutaneous lead, a small tube containing control and power wires that runs through the recipient's skin.
The HeartMate and Novacor devices are self-regulating, according to representatives from the companies. The controllers automatically detect the patient's changing heartbeat and circulatory demands, and increase or decrease blood flow as required. For example, when a patient walks briskly, the natural heart beats faster, and so does the LVAD.
The HeartMate measures approximately four inches in diameter and less than two inches in depth; it weighs in at about two pounds. According to Vic Poirier, chief technology officer for Thoratec, the device is capable of delivering a blood flow rate of 10 liters per minute, which is enough flow to restore critical circulation and reverse organ dysfunction. The average life of the pump is two to three years, depending on the patient, Poirier said.
The HeartMate uses mechanical bearings and is constructed primarily of titanium. The bearings in the cams are made of tooled steel. One of the device's greatest advances is in how it tries to prevent the biggest challenge facing all artificial heart devices: blood clots.
"The natural response of the body to a foreign object is to form clots," Poirier said. Thoratec originally used a smooth surface on its device, which did not allow clots to adhere to it, and used drugs to prevent clotting, according to Poirier. Now, the device uses a special textured surface that is porous. Proteins anchor blood clots to this textured surface, which forms a substance that mimics that of natural arteries, according to Poirier. "With this new textured surface, patients need to take only one baby aspirin a day to control thrombosis," he said.
The Novacor LVAS measures roughly five inches in diameter, and weighs around two pounds, according to Jal Jassawalla, senior vice president of research and development for World Heart. The system is capable of delivering more than 10 liters per minute of blood flow, and has a life span of 4.4 years. According to Jassawalla, nine recipients of the Novacor LVAS were supported by the device for more than three years; three recipients were supported by the device for more than four years.
Like the HeartMate, the Novacor LVAS is constructed primarily of titanium, polyurethanes, and alloys. But the Novacor LVAS does not use a rotary motor, which eliminates some of the complexity of cams, according to Jassawalla. Instead, the Novacor uses a solenoid-based linear driver that attaches to the pusher plate inside the pump. This eliminates one potential source of wear, Jassawalla said.
the littlest LVAD
THE AXIAL FLOW PUMP BEING DEVELOPED by the University of Virginia Artificial Heart Program will serve as the basis for another major research effort. Research associate Amy Throckmorton is leading the effort to develop an LVAD suitable for pediatric use.
Throckmorton's research is focusing on helping children with congenital heart defects. Currently, she's working on developing a continuous-flow centrifugal pump that will be suitable for children from two through 12 years of age. The pump will operate at a lower flow rate than the adult model, pumping on average between two and five liters per minute. The pressure head will also be lower, 70 to 85 millimeters of mercury, as opposed to the 100 mm Hg generated by adult LVADs.
A second-generation centrifugal pump, also under development, will be even smaller so that it will be suitable for implant in newborns through two-year-old patients. This device will measure just 2 inches by 1 inch, and generate 1.5 liters per minute of blood flow at a head pressure of 70 to 75 mm Hg.
The ultimate goal, according to Throckmorton, is to develop an axial flow pump that is small enough to fit into the chest of a newborn and powerful enough to support a child of five or six years old.
The National Institutes of Health has issued a request for proposals to develop pediatric heart-assist devices, and the Virginia Artificial Heart Program was one of the first to respond, according to Throckmorton. The program is hoping to receive a substantial grant that will allow it to continue developing pediatric heart devices. -Gayle Ehrenman
The Strong, Silent Type
The current generation of LVADs and implantable artificial hearts represent a major technological step. However, by all accounts, these devices have a long way to go before they are as mechanically sound and user-friendly as they need to be to be widely accepted as a destination therapy for more than just the sickest of the sick.
All of the current heart device manufacturers are working on modified and next-generation devices that aim to be smaller, more durable, and more powerful, and that operate more quietly.
Researchers at the Artificial Heart Program at the University of Virginia in Charlottesville pioneered the design of a continuous flow centrifugal heart pump. The impeller of this LVAD was entirely suspended by magnetic bearings as opposed to conventional fluid or mechanical bearings, according to Paul Allaire, director of the program. This suspension, which operates on the same principle of magnetic levitation used by maglev trains, allows the impeller to avoid any contact with the internal housing of the pump and provides larger clearance gaps for ease of blood flow, Allaire said. The design also reduces regions of stagnant and high shear flow that normally surround a fluid or mechanical bearing. And, magnetic bearings, because they have no moving parts, do not wear over time, giving the resulting LVAD a long life span, which Allaire expects to be 10 to 15 years.
The Artificial Heart Program's final centrifugal pump design has been licensed to MedQuest Products of Salt Lake City. The device, which Med Quest intends to market as the HeartQuest VAD, is currently undergoing animal testing at the Utah Artificial Heart Institute. Thoratec is currently developing its own magnetic bearing- based centrifugal pump product, called the HeartMate Ill, according to Poirier. This fully implantable device is currently undergoing animal testing, and could be in human tests within two years.
The Virginia Artificial Heart Program has moved beyond centrifugal pumps to work on what Allaire calls "a smaller, less invasive device," the axial flow pump.
According to Allaire, the axial flow pump is a "pencil-size" device that has lower power consumption than current generation devices, as well as the centrifugal pumps. Like the centrifugal pump, the axial flow pump offers continuous flow. Current generation devices, by contrast, are reciprocating devices that use mechanical n1eans to make a membrane go back and forth, producing the thumping that serves as a pulse in an artificial heart.
The continuous flow devices, by contrast, will not be pulsatile. This may take some adjustment on the part of patients, who won't hear a pulse or a heartbeat, but should not cause any negative effects, according to Allaire.
The axial flow pump works more like a small turbine or jet engine than a mechanical pump. It moves blood for ward at very high speeds of roughly 5,000 to 10,000 rpm, Allaire said. The device that the Virginia Artificial Heart Program is working on is roughly a quarter to a half the size of the current generation of reciprocating pumps, according to Allaire. This allows it to draw less power and also fit into a broader range of patients. Current devices are estimated to fit in only 30 percent of the population, according to Allaire. There is no long-term reliability data for continuous flow devices, at this point. Animal trials of the device are currently under way.
Thoratec is developing its own axial flow device, the HeartMate II. Thoratec's Poirier expects that this device, which will have mechanical bearings, will be through clinical trials within the next two years. He expects that the device will have a life span of roughly five years.
Despite all these advances in artificial heart technology, devices will have to be fully implantable, quiet, and reliable before they will be considered for widespread use. In other words, they'll have to be as forgettable as properly functioning natural hearts-operating quietly without calling any attention to themselves. It's a goal that is probably still 10 years away from reality.